We have utilized a signal averaging, high resolution ECG to record His bundle potentials from the body surface of 111 normal Baltimore Longitudinal Study (BLS) volunteers ages 21 to 79. By allowing measurements of conduction time both proximal and distal to the bundle of His, this technique should enhance our understanding of the age-related changes in the cardiac conduction system. In 52 women, neither PR nor HV interval was related to age. In 59 men, the following age relationships were found: PR interval = 142.4 msec + .477 age .388 less than .01 PH interval = 105.3 msec + .427 age .393 less than .01 PR segment = 47.6 msec + .315 age .328 less than .02 Proximal PR segment = 10.5 msec + .267 age .330 less than .02 Thus, an age-related prolongation of PR interval is found only in men and appears to be due largely to a delay in the proximal PR segment, presumably reflecting delay within the atrioventricular junction.